The p53 tumor suppressor gene continues to be distinguished as the most frequently mutated gene in human cancer. Induction of p53 can cause either growth arrest or apoptosis, but it is believed that p53's apoptotic functions largely underlie its selection for inactivation in cancer. Currently, there are several gaps in our knowledge about the p53-dependent apoptosis pathway. While it is known that p53 can transcriptionally activate a number of genes involved in cell death, it is also a transcriptional repressor, and can repress the expression of anti-apoptotic genes. The contribution of transcriptional repression by p53 to cell death has never been determined, and will be a focus of the current proposal. p53-dependent transcriptional repression occurs through diverse binding sites on the promoters of repressed genes. However, we believe we have found a focal point in transcriptional repression by this protein, the p53-Sin3-HDAC (histone deacetylase) complex. The hypothesis that this complex is used as a common mechanism of repression by p53 will be tested. Finally, we have found that the codon 72 polymorphism in p53 alters its transcriptional properties. Specifically, the arginine (R72) variant of p53 has greatly enhanced ability to transactivate a subset ofp53-induced genes, along with markedly increased ability to induce apoptosis. We will test the hypothesis that transactivation of this subset of pro-apoptotic genes is critical for the cell's decision to undergo growth arrest or apoptosis. At the heart of this application is the proline-rich domain of p53. It has been known for many years that deletion of this domain renders p53 impaired for apoptosis induction, but its function in cell death has never been fully understood. Sin3-binding and the codon 72 polymorphism both occur within this domain. Completion of the proposed studies will shed necessary light on the function of the proline-rich domain of p53 in apoptosis induction.